The invention relates to a method for operating an exhaust gas system, in particular of a vehicle, in which measuring values are evaluated which indicate a content of nitrogen oxides in an exhaust gas downstream of a catalytic device. The catalytic device is adapted to diminish the content of nitrogen oxides in the exhaust gas produced by an engine of motor vehicle. Based on the measuring values a quality of a reducing agent is assessed, which is supplied to the catalytic device. The method comprises the step of determining whether reducing agent has been filled into a storage tank. Furthermore, the invention relates to a control assembly for operating an exhaust gas system.
Due to stringent exhaust emissions regulations, engines such as diesel engines for vehicles are generally equipped with an exhaust gas aftertreatment system. Such an exhaust gas aftertreatment system can comprise a catalytic device which is designed as a so-called selective catalytic reduction (SCR) catalyst. The function of the SCR catalyst is to convert nitrogen oxides (NOx) to non-harmful nitrogen (N2) and water (H2O) via a catalytic reaction between the nitrogen oxides and ammonia (NH3).
Ammonia as a reducing agent is supplied to the exhaust gas stream entering the catalytic device by injection of a fluid into the hot exhaust gas, which is known as a diesel exhaust fluid (DEF). The diesel exhaust fluid can in particular be a urea-water solution with 32.5% urea. The injected diesel exhaust fluid releases ammonia through a hydrolysis reaction upon injection into the hot exhaust gas.
The ammonia is first stored in the SCR catalyst. Then the ammonia reacts with NOx molecules on the surface of the SCR catalyst. If ammonia is oversupplied, it may not all be stored in the SCR catalyst and may slip out of the SCR catalyst and get into a tailpipe of the exhaust gas system. Since the slipped ammonia is a nuisance and an odorous gas, the SCR is usually followed by a short section called an ammonia slip catalyst (ASC). The ammonia slip catalyst converts slipped ammonia to nitrogen and water. The dominant reaction is to convert ammonia to N2 and H2O. However, if the ammonia slip amount is too high, some of the ammonia will be converted to nitrogen oxides by the ammonia slip catalyst. With an increase of the ammonia slip into the ASC catalyst an increasing amount of nitrogen oxides can be created. If the ammonia slip is very high, the NOx creation will become significant.
One of the factors critically impacting the effectiveness of NOx reduction is the ratio of the molecules between NOx and ammonia, the so-called ammonia-to-NOx ratio (ANR). There is a narrow ANR range which gives maximum conversion. If the ammonia is undersupplied, the NOx conversion efficiency is reduced, caused by the shortage in ammonia supply. Thus, ammonia undersupply can cause NOx emissions to be incompliant. If on the other hand the ammonia is oversupplied, some of the unused ammonia may slip out the SCR catalyst. If the slippage is high enough, extra NOx can be created in a downstream ammonia slip catalyst.
The undersupply of ammonia can have multiple root causes. One cause could be a dilution of the reducing agent, i.e., the diesel exhaust fluid, which results in a lower ammonia content with the same injection rate. Dilution of the diesel exhaust fluid can be done by water, but is not limited to water only. The diesel exhaust fluid can also be diluted by other fluids which are free of ammonia. If a diluted diesel exhaust fluid is utilized and the diesel exhaust fluid injection rate is not corrected, the NOx conversion efficiency will drop. Consequently, tailpipe NOx emissions may exceed regulated levels.
For the reasons mentioned above, some emissions regulations require the implementation of an on-board diagnostics means to monitor the quality of the reducing agent and to take certain actions once the deterioration of the reducing agent's quality reaches a level which will cause the tailpipe NOx to exceed the regulated limit.
U.S. Pat. No. 8,209,964 B2 describes a method for assessing the quality of a reducing agent which is injected into an exhaust system upstream of an SCR catalyst. A first sensor generates a first signal indicative of an amount of nitrogen oxides entering the SCR catalyst. A second sensor generates a second signal indicative of an amount of remaining nitrogen oxides downstream of the SCR catalyst. A third sensor is utilized to detect whether a change in the NOx reduction efficiency is associated with a fill event of the reducing agent. If this is the case, a dramatic change in the NOx reduction leads to the conclusion that the added fluid is not the reducing agent. If only a moderate drop of the reduction efficiency is observed, the dosing of the reducing agent is increased. If such a reaction results in an improvement of the NOx reduction, the future dosing is adjusted accordingly and the operator is warned that an improper reducing agent has been filled into a storage tank for the reducing agent.
Such a method is quite complex, as three sensors are utilized to assess the quality of the reducing agent. Further, as the method uses absolute sensor signal readings, the method is sensitive to a drifting of the sensor signal. This may lead to an unreliable determination of the reducing agent's quality.
Another approach is to utilize a quality sensor which is installed in the diesel exhaust fluid tank. Such a commercially available sensor can be utilized for quality monitoring and the detection of a deterioration of the quality of the diesel exhaust fluid. Generally, the DEF quality is checked through density sensing. For this, the sensor is dipped into the diesel exhaust fluid in the tank. However, the sensor has to cover the whole depth of the diesel exhaust fluid tank in order to detect a stratified dilution of the fluid in the tank. For example, an ultrasound sensor can be utilized to measure the density of the diesel exhaust fluid. However, there are other types of sensors based on electrical current conductivity, thermal conductivity etc. The signal of such a quality sensor can be utilized to correct the DEF injection rate.
However, installing a DEF quality sensor in a DEF tank leads to increased costs. Further, there are added monitoring requirements for the sensor itself. One of the requirements is the sensor signal's rationality check, which can be very difficult to do as there is no good reference point for comparison. Further, there is potentially a DEF stratification of water and urea after the DEF experiences freezing and thawing at low enough ambient temperature. Under such conditions the signal error increases. Further, the sensor output can be affected by different diluent agents utilized. Such a reaction of the sensor is known as a cross sensitivity.
It is therefore an object of the present invention to provide a method and a control assembly of the initially mentioned kind, which is particularly simple and reliable in assessing the quality of the reducing agent.
In the method according to the invention, a plurality of measuring values is captured during a predetermined period of time, and a magnitude and a frequency of the plurality of measuring values are taken into account to assess the quality of the reducing agent. Thus, the method only requires the evaluation of measuring values of a single sensor indicating the content of nitrogen oxides in the exhaust gas, wherein this sensor is located downstream of the catalytic device. Additionally, means for determining whether reducing agent has been filled into the storage tank are utilized, such as a storage tank level sensor. As it is not necessary to calculate the NOx reduction efficiency by comparing the signals from a first NOx sensor located upstream of the catalytic device with the signals from a second sensor located downstream of the catalytic device, the method is particularly simple. It is sufficient to evaluate the measuring values of the one sensor only, which is located downstream of the catalytic device.
If a decrease in conversion performance of the catalytic device occurs subsequent to a refilling of the storage tank, it can be concluded that the reducing agent filled into the storage tank had an improper quality. Applying a magnitude-frequency analysis to the measuring values or signals provided by the sensor located downstream of the catalytic device thus leads to a particularly simple and reliable way of assessing the quality of the reducing agent. The method thus provides an on-board technique to detect an improper quality of the reducing agent such as a dilution of the reducing agent without a physical quality sensor.
In an advantageous embodiment a magnitude of a measuring value captured within the predetermined period of time is related to an average or to a median of the magnitudes of the plurality of measuring values captured during the predetermined period of time. The related magnitude is then utilized to assess the quality of the reducing agent. As the magnitude is related to the average or the median, a relative magnitude-frequency analysis is utilized instead of an absolute magnitude-frequency analysis. Thus, a particularly high detection resolution is achieved with either a new or a fresh catalytic device or an aged catalytic device.
Further, by utilizing related magnitudes, the method tolerates signal drift and is thus particularly reliable for detecting a deterioration of the reducing agent's quality. By relating the magnitude to an average or to a median of magnitudes, a variation of the absolute magnitude of the measuring values does not have an unwanted influence on the determination of the conversion performance and thus the quality of the reducing agent. For example, an aged catalytic device with a sufficient supply of reducing agent will not be mistakenly detected as a supply of diluted reducing agent which leads to higher absolute magnitudes of the signals or measuring values indicating the content of nitrogen oxides in the exhaust gas downstream of the catalytic device.
In a further advantageous embodiment a moving median is utilized for relating the magnitude of each measuring value captured within the predetermined period of time. In a particularly simple configuration the moving median is a mathematic average of the magnitudes of the plurality of measuring values which are symmetrically arranged around the magnitude or value to be related to the moving median. Thus, the influence of an overall trend of the measuring value magnitudes can be detected and is not falsely interpreted as an inappropriate quality of the reducing agent supplied to the exhaust gas. Thus, utilizing the moving median enhances the reliability of the quality assessment.
It has further proven to be advantageous if in relating the magnitude to the average or median a difference between the magnitude of the measuring value to be related and the average or median is calculated. By taken into consideration this difference, the noise of the measuring values or signals is detected. This is based on the finding that the quantity of the reducing agent stored in the catalytic device has a damping effect on sudden variations in inflowing nitrogen oxides, in particular on sudden increases of inflowing nitrogen oxides. If the level of reducing agent stored in the catalytic device is reduced or if there is no stored reducing agent left at all, the reduced damping capacity to the inflowing nitrogen oxide variations results in the noisiness of the tail pipe nitrogen oxide sensor's signal. Therefore, by taking into account the noisiness a degree of shortage in the amount of the reducing agent supplied to the catalytic device can be readily detected. Such a shortage can be associated with a quality deterioration of the reducing agent occurring after a refilling event during which the reducing agent is introduced into the storage tank.
Further advantageously a total of absolute values of differences is created in utilizing the related magnitude to assess the quality of the reducing agent supplied to the exhaust gas. Such a total of absolute values is particularly easy to handle and has proven to be a very robust detection tool.
A particularly reliable assessment of the quality of the reducing agent supplied to the exhaust gas is achieved, if the total of absolute values of differences is created over a sampling period which comprises a plurality of the predetermined periods of time. The sampling period can in particular be in the range of 2 minutes to 20 minutes, preferentially in the range of 5 minutes to 15 minutes. A particularly good result in the quality assessment is achieved, if the sampling period is about 10 minutes.
Further, it is proven advantageous, if the predetermined period of time is in the range of 5 seconds to 60 seconds, in particular in the range of 10 seconds to 30 seconds. This is based on the finding that a period of time which is too short may result in an inability to reliably detect a dilution of the reducing agent following a refill event. If, however, the predetermined period of time is too long, the sensitivity for noise detection is decreased. Therefore, the length of the predetermined period of time can in particular be about 20 seconds.
Preferably, the reducing agent is assessed to have an improper quality if the sum of differences between a plurality of totals is greater than a threshold value. As a relatively larger value of the total indicates a lower supply of the reducing agent to the catalytic device, increasing totals over a sampling period indicate an improper quality of the reducing agent, in particular a dilution of the reducing agent. This allows for a particularly reliable dilution detection in an open-loop controlled dosing system of the reducing agent. As an example the dilution of the reducing agent can be readily detected by comparing the cumulative incremental change of subsequent totals with the threshold value.
In a further advantageous embodiment, an amount of the reducing agent which is supplied to the exhaust gas is modified. Thus, a correction of the dosing rate of the reducing agent is performed in case a change of the reducing agent's quality is determined. This allows for compliance with emissions regulations despite a deterioration in the quality of the reducing agent.
The reducing agent can further be assessed to have an improper quality if a value which is based on a plurality of modifications of the amount of the reducing agent supplied to the exhaust gas is greater than a threshold value. Such a procedure is in particular useful for closed-loop controlled reducing agent dosing systems. This is based on the finding that the reducing agent can be assessed to have an improper quality, in particular be diluted, if despite the modification of the amount of the reducing agent supplied to the exhaust gas, no or only a limited performance improvement of the catalytic device is observed. In such a scenario, the cumulative incremental change of the value which is based on the plurality of modifications can be utilized to detect a dilution of the reducing agent.
A particularly simple implementation of the method is achieved if a sum of differences between a plurality of correction factors is utilized as the value which is based on the plurality of modifications.
The control assembly according to the invention for operating an exhaust gas system comprises an evaluation unit adapted to evaluate measuring values which indicate a content of nitrogen oxides in an exhaust gas downstream of a catalytic device. The catalytic device is adapted to diminish the content of nitrogen oxides in the exhaust gas produced by an engine. A detection unit of the control assembly is adapted to assess a quality of a reducing agent supplied to the catalytic device based on the measuring values. The detection unit is further adapted to determine whether reducing agent has been filled into a storage tank and to take into account a magnitude and a frequency of a plurality of measuring values captured during a predetermined period of time in order to assess the quality of the reducing agent.
The advantages and preferred embodiments described for the method according to the invention also apply to the control assembly according to the invention and vice versa.
The features and feature combinations mentioned above in the description as well as the features and feature combinations mentioned below in the description of figures and/or shown in the figures alone are usable not only in the respectively specified combination but also in other combinations or alone without departing from the scope of the invention. Thus, implementations not explicitly shown in the figures or explained, but which result and can be generated by separated feature combinations of the explained implementations are also to be considered encompassed and disclosed by the invention.
Further advantages, features and details of the invention are apparent form the claims, the following description of preferred embodiments as well as based on the drawings.